Therapeutically interesting compounds have been isolated from natural sources. These antitumor compounds are often structurally complex, and for this reason systematic chemical studies on the structural and chemical requisites for drug action have been hindered. Microorganisms are known to convert a variety of organic compounds into metabolites which accumulate in fermentations. It is proposed that microbial transformations of naturally occurring antitumor compounds will result in the production of metabolites which will be useful as drugs. Microbial transformations will be developed as a general method for: a) producing quantities of potentially active metabolites of naturally occurring antitumor compounds; b) providing difficult-to-synthesize drug metabolites which will facilitate drug metabolism studies in animals and man; and c) establishing a working basis for further structure-activity relationship studies on complex and biologically active compounds from nature. Studies have been conducted so far on Withaferin-A; acronycine; d-tetrandrine and thalicarpine. Microbial metabolites have been obtained and identified by chemical and spectral methods, and a few of these have been examined for antitumor activity. Fermentation parameters influencing the quantity and number of metabolites have been examined. BIBLIOGRAPHIC REFERENCES: R.V. Smith and John P. Rosazza. 1975. Microbial systems for study of drug biotransformations. Biotechnology and Bioengineering 17,785-814. R.V. Smith and John P. Rosazza. 1975. Microbial models of mammalian metabolism. J. Pharm. Sci., 64, 1737-1759.